The armature of a power generator is made up of a magnetic iron core, referred to as a stator core, that guides the magnetic flux, and conductive windings that carry the current. The magnetic core is cylindrically shaped and made up of thousands of thin laminations of steel clamped together to make a solid structure. The core has axial slots for the conductive windings, and these windings form loops around arcs of the stator core. A loop is formed by a coil that includes the conducting wires insulated from the core so that the wires can maintain a voltage difference from coil to coil and from coil to core. The coils fit into the slots in the core. FIG. 1 illustrates one example of how a portion of the end of a stator core 20 may appear. This figure focuses on a cross section of one slot 28. As shown, the stator comprises many neighboring coils 26 aligned in unison around the shaft, though gaps 30 between the coils and the surrounding abutments are usually tough to avoid.
The insulated coil width must be slightly smaller than the width of the slot to allow the coil to be installed in the slot. Once the coil is installed, it must be tightened in the slot; therefore, side wedges are driven between the slot wall and the coil. Additionally springs and wedges are driven at the top of the slot to keep the coils radially tight in the slot. It is important that the coils are tight in the slot because movement can cause wear of the insulation. Electromagnetic forces that peak twice per cycle exist that would vibrate a loose coil radially. Weaker forces also exist that would vibrate a loose coil circumferentially.
Traditionally side wedges have been composed of the thin sheets of mica or semi-conducting glass epoxy wedges. Each with their own advantages. Mica has advantages in that it is strong yet flexible, and is resistant to the generator environment. Further, due to the flaky nature of mica sheets, if a thin sheet mica wedge begins to lose cohesion through delamination, it will actually expand, further tightening itself within the slot.
Despite the benefits of using thin sheet mica wedges, it has been found that the semi-conducting wedges improve generator performance by dissipating any surface charge from the wound coils. Since mica is such a good insulator, other materials were needed to make semi-conductive thin sheet wedges. One technique is to impregnate a laminated glass fabric with a conductive resin. However, the laminated glass is not as stiff as the mica, and does not make as strong a supportive wedge. Further, under the strong electrical and magnetic conditions of the generator, the resin impregnated glass breaks down; and unlike the mica that actually expands when delaminating, the organic matrix simply disintegrates leaving a loose fragile glass weave.
Efforts have been made to combine the semi-conductive nature of the resin impregnated glass with the strong support of the mica by inserting alternating wedges of the glass and mica. Unfortunately that produces a wedge matrix that is neither fully supportive nor properly semi-conductive.
What is needed is a thin sheet wedge that has the strength and durability of a mica sheet, while also exhibiting the semi-conductive properties of a resin impregnated glass sheet.